So you've been conversing online. You make a quick joke. And suddenly, everybody acts like you are the greatest, meanest Jerkass ever. You get exiled, your spouse wants a divorce and all your friends leave you. And you know why? Because people had taken your joke seriously and felt genuinely offended. And you can't blame them for that. It's pretty easy to mistake jokes for seriousness online.
And to prevent these kinds of misunderstandings, there is Humor Mode. With this, you can mark a text to show that you are, well, just joking. You were not serious at all about what you just wrote.
Compare and contrast with "Just Joking" Justification (as in using this doesn't always work).